The effectiveness of pre- and post-zygotic barriers in avoiding hybridization between two snapdragons (<i>A</i><i>ntirrhinum</i> L.: Plantaginaceae)

Abstract

Reproductive barriers play an important role in the maintenance of species boundaries. However, to date, few studies have provided a detailed analysis of reproductive isolation barriers between species or examined their importance in maintaining species identity. This is the first detailed study into pre- and post-zygotic reproductive isolation barriers in Antirrhinum, based on a mixed population with two species that rarely co-occur. The study revealed that pollinator constancy and preference and poor hybrid seed viability were the most important reproductive isolating mechanisms. Reproductive isolation was practically complete by both pre- and post-zygotic barriers. Average pre-zygotic isolation was greater than post-zygotic isolation, in accordance with the trend observed in flowering plants in which reproductive isolation is principally caused by pre-zygotic mechanisms. However, average post-zygotic isolation was also high, in contrast to what was expected among Antirrhinum spp. This case highlights the importance of quantifying the reproductive isolation barriers thoroughly to understand how and why species boundaries are maintained.